Unique Constraint

What Does Unique Constraint Mean?

A unique constraint is a type of column restriction within a table, which dictates that all values in that column must be unique though may be null.

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To ensure that a column is UNIQUE and cannot contain null values, the column must be specified as NOT NULL. Interestingly, these are a primary key’s two main attributes. Defining both attributes in a newly-created column should be given serious consideration for the primary key designation.

Techopedia Explains Unique Constraint

A unique constraint is defined at the time a table is created. A unique constraint allows null values. Initially, this may seem like a contradiction, but a null is the complete absence of a value (not a zero or space). Thus, it is not possible to say that the value in that null field is not unique, as nothing is stored in that field. A null value cannot be compared to an actual value. For example, the Queen of America cannot be compared to the Queen of England because the Queen of America is a null that does not exist.

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Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.